1. Field of the Invention
The present invention relates to organometallic complexes and an organic electroluminescence device using the same, and more particularly, to organometallic complexes capable of emitting light over a wide range from a blue region to a red region through triplet metal-to-ligand charge transfer (MLCT) and an organic electroluminescence device using the same as an organic layer forming material.
2. Description of the Related Art
Generally, an organic electroluminescent (hereinafter referred to as EL, hereinafter) device is a spontaneous light-emitting display device which emit light by energy generated through recombination of electrons and holes when an electric field is applied to thin films made of fluorescent or phosphorescent organic compounds (to be referred to as organic layers, hereinafter). The organic EL device has good lightness, constructional simplicity, high color purity, perfect implementation of motion pictures, low power consumption, a low driving voltage, and so on, and the organic EL device is suitable to be used for portable electronic devices.
A general organic EL device includes an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer an electron injection layer, and a cathode, which are sequentially formed on a substrate. The hole transport layer, the light-emitting layer, and the electron transport layer are organic layers made of organic compounds. The organic EL device having the above-described configuration is driven as follows. When a voltage is applied between the anode and the cathode, holes injected from the anode migrate to the light-emitting layer via the hole transport layer. Electrons emitted from the cathode are injected into the light-emitting layer via the electron transport layer. The electrons and the holes recombine in the light-emitting layer to generate excitons. While the excitons are radioactively decaying, light with a wavelength corresponding to a band gap of the molecules is emitted.
Materials for forming the light-emitting layer of the organic EL device are classified into a fluorescent material which uses a singlet exciton and a phosphorescent material which uses a triplet exciton, according to a light-emitting mechanism. The fluorescent material or the phosphorescent material forms a light-emitting layer by itself or by being doped to an appropriate host material. As a result of electron excitation, singlet excitons and triplet excitons are produced in the host. Statistically, the singlet excitons and the triplet excitons in an OLED are created in a ratio of about 1:3. Conventional organic EL devices using a fluorescent material as a material for forming a light-emitting layer are disadvantageous in that triplet excitons are consumed from the host. However, conventional organic EL devices using a phosphorescent material as a material for forming a light-emitting layer are advantageous in that singlet excitons and triplet excitons are both utilized to achieve the internal quantum efficiency of 100%. Thus, an organic EL device using a phosphorescent material as a material for forming a light-emitting layer has a high emission efficiency compared with an organic EL device using a fluorescent material.
Introduction of a heavy metal such as Ir, Pt, Rh, or Pd to organic molecules has led to spin-orbital coupling due to a heavy atom effect so that a triplet state and a singlet state coexist, enabling a forbidden transition and phospholuminescence to occur even at room temperature.
More recently, developments have led to the discovery of high-efficiency green and red phosphorescence materials with the improved internal quantum efficiency as discussed by the following articles; Baldo, et al., Nature, vol. 395, 151-154, 1998; Baldo, et al., Appl. Phys. Lett., 75, 4-6, 1999; Adachi, et al., Appl. Phys. Lett., 77, 904-906, 2000; Adachi, et al., Appl. Phys. Lett., 78, 1622-1624, 2001. In particular, a green phospholuminescent (PL) material using fac tris(2-phenylpyridine)iridium (Ir(ppy)3) has an external quantum efficiency of 17.6±0.5%. Bis(2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C) iridium (acetylacetonate) (Btp2Ir(acac)) has been reported as a red EL material having a maximum external quantum efficiency of 7.0±0.5%.
As described above, as highly efficient luminescent materials using phospholuminescence, various materials employing various transition metal complexes containing a transition metal such as iridium or platinum, have been being reported. However, materials satisfying requirements for realizing a full-color display of high emission efficiency or white electroluminescence with low power consumption are only restricted to ones emitting in the green and red ranges, and blue phosphorescent materials have not been reported, making it difficult to achieve a full-color display, which is, in turn, becoming a barrier to development of phospholuminescent full-color display devices.
To address the above-described problems, intensive development of blue luminescent materials is under way (WO 02/15645 A1 entitled Organometallic Compounds and Emission-Shifting Organic Electrophosphorescence and published Feb. 21, 2002, U.S. Patent Publication. No. 2002/0064681 A1 entitled Luminescence Device, Display Apparatus and Metal Coordination Compound and published on May 30, 2002). Also, there have been proposed organometallic complexes having a bulky functional group or a functional group having a high intensity ligand field, e.g., a cyano group, introduced thereto to increase a difference between the energy levels of the highest energy occupied molecular orbital and the lowest energy occupied molecular orbital (HOMO-LUMO energy levels) by transforming the molecular geometry. Another materials that have recently been developed include iridium complexes having the general formula of Ir(ppy)2P(ph)3Y, where Y=Cl or CN, as described in US2002/0182441 A1 entitled Organometallic Compounds and Emission-Shifting Organic Electrophosphorescence and published on Dec. 5, 2002, and iridium (III) complexes having a cyclometalating ligand and chelating diphosphine, chlorine and cyano group, as described in U.S. Patent Publication No. 2002/0048689 A1 entitled Light-Emitting Device and Iridium Complex and published on Apr. 25, 2002.